Piezoelectric materials, especially piezoelectric ceramics such as PZT (Lead Zirconate Titanate) are the most widely known materials used for the manufacturing of stacked actuators and transducers among all ferroelectric-type materials. Recently, however, other materials have begun to be developed for use as stack actuators, which exhibit many beneficial properties not available in traditional piezoelectric materials. One type of these materials is the relaxor ferroelectric single crystals such as piezoelectric lead manganese niobate lead titanate (PMN-PT) and lead zinc niobate-lead titanate (PZN-PT). These relaxor ferroelectric single crystal materials lead to a new generation of piezoelectric materials possessing prominent properties and are poised to advance piezoelectric applications. The attractiveness of these materials lies in the fact that their piezoelectric coefficients (d33>1500 pC/N, d31: 900˜1100), electromechanical coupling factor and strain levels (0.5%˜1.2%) are significantly higher than those of conventional lead zirconate titanate (PZT) materials. Unlike piezoceramic and sol-gel film piezoactuators that employ strain magnification schemes, single crystal actuators can deliver higher strain levels without sacrificing generative force. Further, the low strain hysteresis and the stability of single crystals result in improved controllability for piezoactuators.
Such single crystal piezoelectric materials are typically manufactured by crystal growth technique and its ingot is then sliced into wafers with a thickness usually thicker than two hundred microns. However, in order to use the high performance single crystal piezoelectric material in producing its layered stack actuators, there are a few major technical barriers to overcome. Firstly, it is difficult to make bulk single crystal piezoelectric sheets with a thickness less than one hundred microns with no physical cracks, and as a result, the thickness of the single crystal piezoelectric sheets used in stacked actuators are usually from several hundred microns to several millimeters. Secondly, bonding of multiple layers of such wafers in order to achieve a multi-layer stacked wafer is technical challenging since the material becomes extremely fragile within such thickness range, and as a results, the stacked actuators made of such single crystal piezoelectric materials are usually offered with relatively thicker thickness for each of its layers and the overall size of a stack transducer is typically large. Thirdly, in order to manufacturing stacked actuator in an arrayed production manner, there is a stringent need to make electrical connections from segmented actuators' sidewalls since the overall stack thickness becomes extremely challenging to achieve full coverage of deposited metal material(s), and this is especially true when tens to hundreds of piezoelectric layers are stacked. Lastly, the technical requirement becomes even more stringent if an array of stack micro actuators is being manufactured wherein the actuator density per unit area is significantly higher than that of macro-sized stack actuators. As a result, the manufacturing of single crystal stack actuators, which are used for actuating deformable mirrors for correction of wavefront aberrations for adaptive optics, are highly limited in achieving high performance with regards to a plurality of key parameters including, but not limited to sizes, array densities, strokes, actuation voltage requirement, manner of implementing associated driver electronics, and the overall device weight and cost.
In addition to relaxor ferroelectric single crystal wafers, other piezoelectric and/or electrostrictive materials such as traditional PZT ceramics, PZT single crystals, PMN ceramics, etc. are facing the same challenging in manufacturing stack actuators in order to drive motions of a deformable mirror when they are offered as bulk wafers with relatively thicker thickness, thus calling for a solution that is applicable to all above-mentioned wafer materials.